Cooking oil is extensively used in restaurants and in the food preparation industry for cooking of various food items. One common use for cooking oil is in the cooking process known as deep-fat frying. Such frying is frequently carried out in relatively deep containers of cooking oil, with the food item to be cooked being immersed in cooking oil that is heated to a temperature between about 250° F. and about 400° F. (about 121° C. and about 205° C.), and in most cases, to a temperature between about 315° F. and about 385° F. (about 158° C. and about 196° C.). As food items are introduced into the fryers and cooking proceeds, the cooking oil becomes contaminated with suspended food particles, blood, water, flour, breading, spices and other introduced contaminants. These contaminants will react with each other and with components of the cooking oil to degrade the cooking oil, resulting in a change in color, a change in alkalinity or acidity, a reduction in thermal efficiency and an increase in the potential for smoking or burning. In addition, dispersed and suspended particulate will conduct heat away from the food product, increasing frying times and heating energy requirements. Furthermore, the exposure of these introduced contaminants to the air and the heat of cooking over time will cause suspended particles to become charred or carbonized. This carbonization is frequently accompanied by chemical breakdown of the cooking oil, resulting in the production of impurities such as free fatty acids, polymers, alkalines and other polar compounds. All of these contaminants can cause the effectiveness of the cooking oil to degrade. Furthermore, such contaminants may attach themselves to food products during the frying process, thereby raising the potential for adversely affecting the taste of the food products and/or creating negative health issues upon consumption.
Restaurants and food preparation facilities frequently fry food items in batches, and they generally do not discard the cooking oil after cooking a single batch. Consequently, filtration is commonly employed to control or reduce contaminants and to extend the useful life of the cooking oil. In typical restaurant operations, the cooking oil in a deep-fat fryer is filtered once or twice each day, most often by employing a portable batch-type filtration device. However, despite such filtration, a typical restaurant facility that utilizes a conventional filtration process generally discards its cooking oil every five to ten days due to accumulated particulate contamination and degradation.
Conventional filtration devices generally operate by draining the cooking oil from the fryer into a filtration container and then cycling the cooking oil through a filter. Generally, when the filtration operation is begun, the cooking oil is at or near cooking temperature, and batch filtration continues until a subjective determination is made that the filtering process has achieved a desired result. After filtration, usually comprising multiple passes through the filter, the filtered cooking oil is returned to the fryer. It is not unusual for cooking oil to be filtered by passing it through a conventional filter for 30 minutes or longer. Depending on the amount of time elapsed during filtration, the cooking oil may be as much as 285° F. (140° C.) or more below cooking temperature when it is returned to the fryer. Consequently, the filtered oil must be heated to raise it to cooking temperature prior to resuming frying operations.
Particulate separation is the essence and purpose of filtration. The efficiency of a particular filter material is measured by the size of particulate material that it can retain, the overall amount of particulate that can be retained, and the volume of filtrate that can flow through the filter in a given period of time at the operating pressure of the filtration device. Conventional filters often become laden with particulate material from the frying process in high volume operations, thus making it difficult to maintain an adequate flow of cooking oil through the filter. Under such circumstances, the speed of filtration is greatly reduced. Consequently, fryer operators may attempt to scrape the filter surface to reduce the accumulated particulate to increase the filtration flow.
Conventional filters used in filtration devices include those comprised of paper, a pad of bonded fibrous material such as cellulose fibers bound by a resin, and metal screens. Some filtration devices employ flat filters that are retained in a support frame; others employ filter envelopes that wrap around a spacing grid or frame. Conventional filter media types vary in their effectiveness. Paper filters are inexpensive; however, paper filters are fragile and are frequently damaged if scraped to remove accumulated particulate. Paper filters cannot generally be used for multiple filtration cycles. Consequently, if a restaurant filters its cooking oil twice a day using paper filters, it will most likely use two filters each day. Paper filters absorb cooking oil during the filtration process, and they provide limited separation efficiency, retaining particles within the range of 20-30 microns and larger. Cellulose filter pads also absorb cooking oil during filtration, and they are susceptible to damage if scraped. Cellulose pads are difficult to form in the shape of a filter envelope that wraps around a spacing grid, and are not usable for multiple filtration cycles. However, cellulose filter pads generally retain particles within the range of 1-5 microns and larger. Generally, paper filters and cellulose filter pads may be used to filter approximately 180-270 lbs (397-595 kg) of cooking oil before they must be replaced. Stainless steel filter screens are durable and may be reused indefinitely, but they are considerably more costly than paper filters and filter pads. Furthermore, they also provide limited separation efficiency, retaining particles within the range of 80-120 microns and larger.
Conventional filters generally provide passive surface filtration, in which the cooking oil is drawn through the filter surface by vacuum, retaining particles from the oil on the filter surface. In order to provide a measure of depth filtration, cooking oil to be filtered must be circulated through paper filters, cellulose pad filters and metal screen filters for 3-5 minutes in order to build a filter cake of particulate to effectively enhance the separation efficiency. The creation of such a filter cake provides interstitial sites for retaining particulate during subsequent filtration. Frequently, a powdered filter aid is used with conventional filter materials to increase filtration efficiency, primarily by increasing depth filtration. Such filter aids are dispersed in the cooking oil and form a powder cake on the filter surface (along with accumulated particulate contaminants) to increase the filtration surface area, and thereby enhance the removal of relatively small particles. Such filter powder, when accumulated as a filter cake on the filter, provides a plurality of channels permeable to liquid, yet more effective in mechanically filtering small particulates. The addition of a powdered filter aid thus provides additional depth filtration and generally results in the removal of smaller particles than can be removed by filtering through paper, cellulose pad or metal screen filters alone. For example, a use of paper filters in conjunction with a powdered filter aid will generally result in the retention of particles within the range of 1-5 microns and larger. A use of stainless steel filter screens in conjunction with a powdered filter aid will generally result in the retention of particles within the range of 5-10 microns and larger. A use of cellulose filter pads in conjunction with a powdered filter aid will generally result in the retention of particles within the range of 1-5 microns and larger.
Filter aids may also include adsorbents or neutralizing agents to provide active filtration in the form of chemical reaction or electrostatic bonding with contaminants, including free fatty acids and polymers, on a molecular level. In some instances, filter aids may be impregnated in filter paper or filter pads. However, a portion of the smaller particles of the filter powder will generally pass through the filter and remain in the cooking oil after filtration. Such particles may adhere to food items during the frying process, and they may react with components of the cooking oil at cooking temperatures to increase oil degradation. Filter aids can also alter the pH balance of the cooking oil. In addition, powdered filter aids, as well as paper and cellulose pad filters, absorb a certain amount of cooking oil during the filtration process. Since filter aids are discarded along with paper or cellulose pad filters when such filters are changed, or are washed from metal screen filters after each filtration cycle, the oil that is absorbed by the filter aids is discarded (along with the filter aid) after every filtration cycle. Such oil must be replaced periodically to insure proper frying operation.
Examples of filter aids include organic compounds such as diatomaceous earth, adiaphorous clay or pearlite, and silicates such as calcium silicate, aluminum silicate and magnesium silicate. Other filter aids include activated carbon, which may be employed to remove objectionable colors from the cooking oil and eliminate odor-causing components, and alkalis, which may be added to increase the pH of the cooking oil.
Generally, the amount of contaminants removed during the filtration process depends on the type of filter material used, the type and extent of filter aids used and the filtration cycle time.
Although filters of polyester and nylon felt materials have been employed for solid-fluid separation in wet and dry applications, the use of such materials for filtering cooking oils has not been practiced, especially as a part of a vacuum filtration process, because the relatively high cooking temperatures involved are generally at or above the recommended application temperatures for such materials. A survey of published information of manufacturers or distributors of non-woven polyester felt material, including Lantor Advanced Materials Group, National Nonwovens, Western Nonwovens, Inc., Southern Felt Company, Inc., Knowlton Specialty Papers, Inc., American Industrial Felt & Supply, and Sutherland Felt indicates that all of these manufacturers and distributors do not contemplate any application (filtration or otherwise) of polyester felt material in a high temperature environment such as is typically seen in frying operations. Furthermore, such manufacturers do not encourage the use of polyester felt materials in filtration applications (either wet or dry) at temperatures commonly encountered in deep-fat frying. Thus, for example, Sutherland Felt Company, a manufacturer of synthetic and other felts, recommends a filter application temperature no higher than 149° C. (300° F.) and American Felt & Filter Company recommends a continuous application temperature (in dry applications) no higher than 132° C. (270° F.).
It would be desirable if a filter material could be provided for deep-fat frying operations that would be more efficient than conventional filter materials. It would also be desirable if such a filter material could be provided that would increase the useful life of cooking oils used in such operations.